The invention relates to scanning probe microscopes, and particularly to a scanning probe microscope in which image resolution is achieved without a corresponding reduction in scanning speed.
In scanning probe microscopes (SPM's), such as scanning tunneling microscopes (STM's) and atomic force microscopes (AFM's), a probe tip or sensing element is suspended in relation to a sample surface by means of a PZT translator or the like. The probe tip generates a z position error signal which varies in accordance with the distance between the sample surface and the tip, or according to the atomic force applied to the tip (in the case of an AFM). Scanning probe microscopes are capable of revealing surface features of molecular size, far smaller than those observable by most other measurement techniques.
The interaction between the probe tip and the surface of the sample produces useful measurable variations representing the distance between the sample surface and the tip or the force applied by the sample surface to the tip only for a very restricted range of tip-to-surface distances or forces. For this reason, scanning probe microscopes usually employ several mechanisms to maintain the tip-to-sample signal at an approximately constant signal level. The surface topography then is assumed to be the same as the x,y,z coordinates of the servomechanism which varies the relative position of the probe tip and the sample by scanning in the x and y coordinate directions and using the servomechanism to cause the probe tip to "track" the sample surface in the z direction.
FIG. 4 shows the closest presently known prior art. In FIG. 4, the main components include a digital computer 30, with functional blocks 12 and 15 therein. Block 12 algebraically sums a digital "set point" on bus 11 and a filtered probe signal on bus 13A and produces the result (difference) as a digital error signal on bus 14. Block 15 functions as a digital compensating element to slow down the servo loop response to allow time for the mass of a PZT piezoelectric transducer/actuator 21 and the mass of a stage and sample or a probe carried by it to respond in a conventional manner to z control signal 20. Functional blocks 12 and 15, digital-to-analog converter (DAC) 17, amplifier 19, actuator 21, sample 23 (the topography of the surface of which is to be measured), probe 25, analog-to-digital converter (ADC) 27, and filtering function 31 constitute a typical servo loop.
Note that although algebraic summing function 12 and compensation function 15 are implemented inside digital computer 30, both could be performed by hardware, rather than software.
The set point is applied via digital bus 11 to algebraic summing function 12, and the difference between set point 11 and digitized and filtered probe signal 13A is equal to the error signal on bus 14. The servo loop adjusts actuator 21 so that the error signal 14 approaches zero, so that digitized probe signal 26 ideally remains equal to set point signal 11.
In FIG. 4, the servo loop signal amplitude and phase parameters are set to cause actuator 21 to follow rapid changes in the relative z position of probe 25 and the surface of sample 23 so as to maintain the error signal 14 as close to zero as possible. The z control signal 20 input to actuator 21 is determined by the compensated digital error signal 16 applied to the inputs of DAC 17. The z coordinate 16 input to DAC 17 also is input to a display system 29 in computer 30 that displays the surface topography of sample 23 as x, y scanning and z direction tracking by probe 25 progresses.
A major problem with the prior art system of FIG. 4 is that it can be relatively slow, that is, its image resolution is poor due to the relatively high mass which has to be moved by actuator 21 and the speed with which that mass needs to be moved, especially if the surface of sample 23 is very rough. If reasonably fast profiling of the sample surface is desired, the above mentioned delay in movement of the mass in response to z control signal 20 causes a loss of resolution of the sample surface image.
Scanning probe microscopes of the type shown in FIG. 4 are capable of resolving features as small as a tenth of a nanometer, which is small enough to show detail at the scale of atomic lattices. Servomechanism loop parameters are adjustable to allow scanning distances of hundreds of micrometers, while still retaining resolution of a few nanometers. However, the technique illustrated in FIG. 4 has the primary disadvantages that the amount of mass moved by the servomechanism may be large enough (especially if the sample stage and sample are supported thereon as in FIG. 3A) to limit the response speed of the PZT actuator, thus either limiting the scanning speed or blurring the features of the sample surface being scanned due to delay between actuator response and the digital input 16 to DAC 17. Furthermore, the effect of the compensation function 15 is likely to further modify the surface topography information, causing a further loss in resolution. The simultaneous requirement for high resolution and a large scan range typically requires a large number of digitization levels in the servo loop, increasing the cost of the instrument and potentially further reducing scanning speeds, especially if the summing, compensation, and filtering functions all are performed by a digital computer.
Another prior approach to SPM implementation is structurally very similar to that shown in FIG. 4, except that 1) the servomechanism loop parameters are set so that actuator 21 responds only to very slow changes in error signal 14, and 2) the output 13A of ADC 27 rather than the input of DAC 17, is input to display system 29. Dotted line 13A in FIG. 4 illustrates this alternative. Consequently, the scanning surface-to-probe tip interaction (distance or force) is not maintained constant and instead is a dynamically varying parameter representing varying sample surface features. This implementation results in very high resolution of the surface image at high scanning speeds for small surface features, but is severely limited by the short range of the tip to sample surface interaction. As a practical matter, this implementation is not as commonly used as the other.